1. Field of the Invention
The present invention relates to a white light emitting device, and more particularly to a phosphor-converted white light emitting device with low deviation of correlated color temperature and color coordinates, and preparing of the same.
2. Description of the Related Art
From normal using of LED light source to see, the leading factor that affects lifetime of the light source is heat. It is an important problem that LED lighting current faces. Due to the heat, the LED light source may degrade in brightness and change in color over time. The phosphor utilized for the wavelength conversion ages over the lifetime of the light source. This aging results in a decrease in the efficiency with which the activation light is converted. Hence, the ratio of blue to yellow light changes over the lifetime of the device as the phosphor ages. And this leads to an undesirable color shift in the light source.
As discussed in U.S. Patent Pub. No. 2007/0247829, a prior art white LED comprising a blue primary source and the phosphors YAG:Ce combined with dark-red sulfide phosphor (Sr, Ca) S:Eu, shows a considerable amount of color drift after 1000 hours due to the unstable phosphor. This results in its color temperature being altered from 3275 to 3575 K, as depicted in FIG. 7 of the patent reference. Correspondingly, the drift in the y color coordinate over 1000 hours operating time at 85° C. and 85% relative humidity is also considerable, cf. FIG. 8 of the patent reference.
Although a general multichip RGB with proper dominant wavelengths and proper intensity proportions can provide easy color management, it is still not easy to stabilize a specific chromaticity over time while LED junction temperatures change from ambient temperature of 30° C. to 120° C. or higher because individual LED exhibits different thermal dependencies.
To deal with these thermal issues, one may use optical and thermal feedback or feed-forward circuit to maintain the chromaticity to within one MacAdam ellipse, especially if the luminaire is being dimmed while the LED junction temperatures vary rapidly. Nevertheless, the approach is too expensive to be adopted in practice. There is a need to find a better solution for a stable white LED with low deviation of correlated color temperature and color coordinates.